@mixin public($size: 1px, $color: red) {
  border: $size solid $color;
}

.a {
  @include public(2px, green);
  font-size: 1rem;
}

.b {
  @include public(1px, blue);
  color: red;
}

.c {
  // 默认值
  @include public;
  background: yellow;
}
